Challenger Deep consists of three sediment-filled basins.
These western, central, and eastern basins form an elongated depression at the southern end of the Mariana Trench in the western Pacific. They are arranged roughly from west to east and are separated by underwater mounds, rather than forming one simple, uniformly flat hole in the seafloor.
Each basin is several kilometres long and about 2 kilometres wide. The western and eastern basins have recorded depths exceeding 10,920 metres, while the central basin is slightly shallower. Modern sonar mapping and deep-diving expeditions have shown that the basins include slopes, rock piles, and sedimentary ooze.
The feature was first located during the 1872–1876 expedition of HMS Challenger and later named after the survey ship. In January 1960, the bathyscaphe Trieste became the first crewed vehicle to reach the bottom. The number three refers to Challenger Deep’s mapped sub-basins, not to the number of people or expeditions that have visited it.